U.S. Pat. No. 6,400,100 has already disclosed a high pressure lamp and associated operating method for the resonant operation of high pressure lamps in longitudinal mode, and an associated system. A method is specified there for finding the second longitudinal acoustic resonant frequency. It proceeds from the fact that during the continuous traveling of the frequency that excites the longitudinal mode, the resonant frequency in a vertical operating position can be found through the occurrence of a relative increase in the operating voltage of the lamp. It emerges that this method can be used to find the longitudinal frequency for a segregated arc state in vertical resonance, and then to retain it. However, depending on the filling composition of the metal halide filling and on the instant in the sequence of the search procedure, the frequency thus found can be placed substantially too high such that an excitation of the acoustic resonance at the frequency found with the aid of the abovenamed method results in an inadequate mixing and does not neutralize the segregation sufficiently well. Moreover, implementation in an electronic ballast is expensive. Further documents that are concerned with reducing the segregation by targeted excitation of the second longitudinal mode are, for example, US 2003/117075, US 2003/117085, US 2005/067975 and US 2004/095076. Use is made in all these documents of a ceramic discharge vessel with a high aspect ratio of at least 1.5, and which is cylindrical. The ends are straight or hemispherical.
Similar attempts have already been carried out with the aid of known metal halide lamps that still exhibit the previously customary discharge vessel made from silica glass, see U.S. Pat. No. 5,773,937. An attempt had been made here, as well, to use as straight as possible a discharge vessel with a cylindrical middle part. Only of necessity are the ends rounded off and then lead to the pinch seal. Ceramic discharge vessels are described there as well, but are obviously designed for sodium discharge lamps.
U.S. Pat. No. 6,469,442 and US-A 2004/104677 disclose a ceramic discharge vessel having beveled end parts. However, it is not operated in resonant mode.